The present invention relates to methods and apparatus for commencing operation of a solid polymer fuel cell system. In particular, the invention relates to starting fuel cell systems that include a reformer.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as transportation applications and stationary power plants. In some of these applications, the fuel cell system may operate more or less continuously for long periods, albeit at varying power levels. However, in other applications, the fuel cell system may be subjected to frequent on-off cycles and hence go through numerous starts from a shutdown condition. Automotive applications are an example of applications with such a duty cycle.
In general, electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Solid polymer fuel cells operate at relatively low temperatures (circa 80xc2x0 C.) compared to other fuel cell types.
A broad range of reactants can be used in electrochemical fuel cells. The oxidant is typically oxygen, delivered in a substantially pure oxygen stream or in a dilute oxygen stream such as air. The fuel is often molecular hydrogen, delivered as substantially pure hydrogen gas or in a hydrogen-containing gas stream such as a reformate stream. Other fuels, besides molecular hydrogen, may be oxidized directly at the fuel cell anode. For example, methanol, dimethyl ether, and methane may be delivered to the fuel cell anode where they are oxidized to produce protons. Such fuels may be delivered in gaseous streams. For methanol and dimethyl ether however, aqueous liquid streams are more commonly used.
A given solid polymer fuel cell can be expected to operate to some extent on most fuels, either in the gas phase or liquid phase, and therefore provide power. However, the design and operation of a solid polymer fuel cell system is typically adapted for the specific type of fuel stream (both the fuel and phase) which is to be used. Along with differences in the subsystems external to the fuel cells (e.g., fuel circulation, cooling, and/or humidification subsystems), there may also be differences in the fuel cells themselves. At this time, for instance, the anodes in direct methanol liquid feed fuel cells (i.e., cells which operate xe2x80x9cdirectlyxe2x80x9d on unreformed aqueous methanol) typically employ different electrocatalysts and different electrode structures than do fuel cells supplied with hydrogen gas. Another difference between hydrogen gas and direct methanol liquid feed fuel cells might be the choice of solid polymer membrane. In direct methanol liquid feed fuel cells, there is often a problem with crossover of methanol fuel from the anode to the cathode side through the membrane. Improvements in crossover characteristics of membrane materials can be expected to lead to different membranes being preferred for each fuel cell type.
Hydrogen gas is presently a preferred fuel insofar as fuel cell operation and performance (output power) are concerned. However, it can be significantly more difficult to store and handle hydrogen than other fuels. Accordingly, in many fuel cell systems, a hydrogen-containing gaseous fuel stream is created from another fuel using a fuel processing subsystem. Typically, the fuel processing subsystem includes a reformer which generates a hydrogen-containing reformate stream from a fuel feedstock (such as methanol or natural gas), usually by reacting the fuel with steam at elevated temperature in the presence of a suitable catalyst. The fuel processing subsystem also typically includes various other components to assist the reforming process, to purify the reformate stream, and/or to introduce other desirable compounds into the gas stream (e.g., vaporizer, shift converter, selective oxidizer, hydrogen separator, humidifier, etc.).
While reformer-based fuel cell systems are preferred in some applications, there are some difficulties associated with the use of reformed fuel. For instance, aside from the need for and complexity of the fuel processing subsystem itself, it can be significantly more complicated and time consuming to start up the system. Both the solid polymer fuel cells and the reformer typically operate above ambient temperature and thus generally need to be heated before normal operation can begin. The reformer in particular may need to be heated to several hundred degrees Celsius and this can take several minutes to accomplish. Further, during warm-up, operation of the reformer is usually not as efficient and any reformate produced may contain large quantities of impurities such as carbon monoxide which can poison the electrocatalysts typically employed in fuel cell anodes. Thus, any reformate produced during the start-up period may not be of much use for purposes of generating electrical power from the fuel cells. Additionally, the power output of the fuel cells themselves may be relatively low until they have reached a certain operating temperature. Finally, any water supply used in the fuel processing subsystem or in humidification of the fuel cell reactant streams is subject to freezing when ambient conditions fall below 0xc2x0 C., and thus represents an additional potential difficulty for system start-up. As a result, additional subsystems may be required to provide power and/or heat just during the start-up of a reformer-based fuel cell system. For instance, fuel from the fuel feedstock supply can be burned to heat up the reformer. Once the reformer is operating, hydrogen-containing reformate is available to start up the fuel cells. Also, the reformer may be used to heat the fuel cells. This procedure however may be undesirably slow for some applications. Alternatively, a supply of substantially pure hydrogen can be maintained in the system simply for start-up purposes. The hydrogen can be combusted (by burner or catalytic combustion) to provide heat for warming up the reformer and fuel cells. Hydrogen can also be directed to the fuel cell anodes to initiate operation of the fuel cells until a suitable supply of reformate is available. The supply of hydrogen can be stored, for example, as bottled compressed gas or absorbed in metal hydride compounds. However, the hydrogen supply must periodically be replenished. In another approach, reformer-based fuel cell systems can be started up using energy provided by storage batteries or using combinations of the preceding methods.
Direct methanol fuel cell systems (DMFCs) are not subject to the same problems relating to start-up. Direct methanol fuel cells show relatively good performance during the start-up phase and thus are capable of fairly rapid start-up and can provide some useful power output when starting from ambient temperatures. Further still, methanol has a freezing point that is well below the typical lower temperature limit to which the system is exposed in most applications. Thus, methanol and certain aqueous methanol mixtures may not pose a freezing concern (although typical aqueous methanol mixtures for DMFCs are too dilute to provide significant protection against freezing). However, at this time at least, the performance and efficiency of direct methanol fuel cells is not adequate to supplant reformer-based fuel cell systems in all applications.
A solid polymer fuel cell system which comprises a supply of fuel and a reformer can be started up quickly by including a portion of fuel cells in the system that operates directly on a starting fluid comprising the unreformed fuel during a start-up period. Thus, within the plurality of fuel cells in the complete system, at least a first portion provides output power during the start-up period. A second portion of the fuel cells in the system, e.g., the remaining fuel cells, operate on reformed hydrogen-containing gas produced by the reformer after the start-up period, and thus provide output power after a start-up period. The first portion of fuel cells are thus xe2x80x9cstarter cellsxe2x80x9d for the system.
Typically, the reformer is a part of a more complex fuel processing subsystem which includes means for producing a suitable feedstock (e.g., a mixture of fuel and steam) for the reformer. The feedstock is reformed, and may subsequently be purified and/or humidified, to create a stream of hydrogen-containing gas which is then directed to a second portion of fuel cells at normal operating temperatures. The starting fluid may also be a fuel mixture (e.g., a mixture of fuel and water) but it is not reformed. Instead a starting fluid stream comprising the fuel is directed to the first portion of fuel cells during start-up, and the fuel is oxidized directly at the starter cell anodes in the fuel cell system.
In principle, any fuel can be employed that is suitable for oxidation both directly (unreformed) and indirectly (after reforming). Suitable fuels may be gaseous or liquid and include for example methane, ethers such as dimethyl ether, and alcohols such as methanol. A preferred fuel however is methanol. Aside from being relatively plentiful, inexpensive, and suitable for use as a direct and indirect fuel, methanol and aqueous methanol mixtures have freezing temperatures below that of water.
It can be advantageous to adapt the xe2x80x9cstarter cellsxe2x80x9d for operation on the starting fluid stream. In that way, performance during the start-up period is enhanced. Accordingly, the construction and composition of the starter cells (in the first portion) may differ from that of the fuel cells in the second portion. For instance, if the fuel is methanol, it is advantageous to employ an anode electrocatalyst in the first portion of starter cells that is different than the anode electrocatalyst in the second portion of cells. Further, it can be advantageous to employ a membrane electrolyte in the first portion of starter cells that is different than the membrane electrolyte in the second portion of cells.
By adapting the starter cells for direct operation on the starting fluid stream, their performance will be improved on the starting fluid stream but may be worse on the hydrogen-containing gas stream. Nonetheless, after the start-up period is over, it may still be advantageous to direct the hydrogen-containing gas stream to the starter cells (first portion) in order to obtain additional output power therefrom. Alternatively, the starting fluid stream may continue to be directed to the starter cells after the start-up period. In a like manner, even though the second portion of cells may not be adapted for operation on the starting fluid stream, it may still be advantageous to direct the starting fluid stream to the second portion during the start-up period in order to obtain additional output power therefrom. Thus, some or all of the fuel cells may operate initially on the starting fluid stream and then on the reformed hydrogen-containing gas stream.
The first and second portions of the plurality of solid polymer fuel cells may comprise separate fuel cell stacks (i.e., one or more stacks comprising the starter cells and one or more stacks comprising the remaining cells). On the other hand, the first portion of fuel cells may instead be interspersed among the second portion of fuel cells.
During the start-up period, the starter cells may provide enough output power to be useful in heating the reformer, in heating the second portion of fuel cells, and/or in powering a peripheral subsystem (e.g., an air compressor). Compared to starting up all the cells in the system at once, less input energy is required to start only a first portion of them. Then, the starter cells can be used as a source of energy to bootstrap and complete the start-up process.
In preferred embodiments of the method, the start-up period is typically completed when the temperature of a component in the fuel cell system reaches a pre-determined threshold value. Thus, the temperature parameter of the component may be monitored and used to trigger an end to the start-up period of the system. Since the operating temperature of the reformer is generally indicative of its ability to produce a satisfactory reformed fuel stream, its temperature may be used as the trigger in preferred embodiments. Alternatively, the temperature of the second portion of fuel cells may be used as the trigger.
In the preceding, if the characteristics of both the reformer and the fuel cells permit, preferably the starting fluid and the feedstock mixtures would be the same, thus making it possible to store both in a common reservoir.